What makes a shark a shark?

Mary Collins School teacher Blythe Shelley touching a leopard shark at the Aquarium of the BayThat was the question put to a group of Bay Area teachers-all participants in Watershed Week, The Bay Institute's annual back-to-school teacher-training institute, facilitated by our Students and Teachers Restoring a Watershed (STRAW) Project. At the Aquarium of the Bay, these teachers-turned-students got to see, touch, and learn about some of the creatures that live under that Bay-including the Bay's sharks. They also learned about the Aquarium's shark tagging program, which aims to help us better understand these amazing and elusive animals.

So, how do the Bay's leopard sharks, soupfin sharks, sevengill sharks, spiny dogfish, and other shark species differ from "non-shark" fishes? Here are a few key distinctions:

#1. You could say that sharks don't have a bad bone in their bodies. In fact, sharks don't have any bones in their bodies. Sharks-along with their relatives skates, rays, and ratfish-belong to a diverse class of fish that have cartilaginous skeletons, unlike the bony skeletons of other fish.

#2. Body shape. If you look at most fish head on, they have a generally oval shape. Sharks, in contrast, tend to be more triangular with a wide, flat under-surface. Their broad pectoral fins give them lift as they move through the water, not unlike the wings of an airplane. This hydrodynamic shape is key to keeping sharks afloat (you'll see why as we move on to difference #3).

#3. Besides bones, sharks lack the air-filled swim bladders that most fish use for buoyancy (If sharks are airplanes, does that mean bony fish are hot air balloons?) Instead, sharks keep afloat with the help of a large, low-density liver, their unique body designs, and the physics of forward motion. If a shark stops swimming it won't necessarily drown-only some sharks need to swim to breath-but it will sink!

#4. While most fish have gills tucked behind a bony flap called an operculum, sharks exhale water through gill slits located behind their head. Five gill slits are typical, but some sharks -like the sevengill shark found in the Bay-have more. Most sharks use ram ventilation to breath, swimming constantly with their mouths open to keep water flowing over their gills. Bottom dwelling sharks, whose mouths may be buried in the sand, inhale water through an opening on the top of their head called a spiracle and pump water past their gills.

#5. A shark's skin is covered with tiny dermal denticles that differ from scales on most fish. As their name indicates, they bear a physiological similarity to teeth. Their unique structure helps reduce drag as the shark moves through the water-in fact, sharkskin helped inspire the high-tech swimsuits we saw at the Summer Olympics.

#6. Most fish spawn by releasing large numbers of unfertilized eggs and sperm into the water. Sharks, in contrast, reproduce via internal fertilization. Depending on the species, they then lay a much smaller number of fertilized eggs, or carry the eggs inside until they hatch, giving birth to live pups.

Old Adobe Elementary teacher Juliet James examining shark teethSadly, these unique creatures are declining all over the world due to overfishing, pollution, loss of habitat from coastal development, and climate change. And that's bad news not just for sharks but also for their ecosystems. Like lions and wolves, most sharks sit atop the food chain as apex predators; thus their disappearance can trigger a cascade of disruption up and down the chain.

Before moving to California almost five years ago, Ann served as Sally Brown Fellow in Environmental Literature at the University of Virginia, where she taught undergraduate seminars on literature and the environment and coordinated an ongoing reading series featuring nationally prominent nature writers. Prior to that, she spent a year as a research assistant at the Smithsonian Tropical Research Institute's field station on Barro Colorado Island, Panama, studying how young leaves defend themselves against herbivores.